1. Field of the Invention
The present invention relates to methods and apparatus for optically writing and/or reading information on optical discs and more specifically to improvements whereby focus of write/read light on the information surface of the optical disc is corrected passively with respect to thickness variations of the optical disc.
2. Description of the Prior Art
The currently preferred optical disc technology employs disc elements with spiral or concentric tracks of minute (usually on the order of a micron or less in size), optically-detectable marks. One real-time mode of recording (writing) such marks is by scanning the disc with a small focused spot of radiation (e.g., from a laser) which is modulated "off or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with an unmodulated, equivalently small, tightly focused light spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the modulated read-out light reflected from the recorded disc.
In order to write and read information in the form of such minute markings, optical systems of relatively high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the proper positional relation between the writing or reading optical system and the optical disc surface must be stringently maintained. One approach has been to utilize highly-precise turntables for supporting and rotating the optical discs in a predetermined plane. Such apparatus is expensive, but even with such costly turntables it is usually necessary also to provide complex focus-servo devices which effect lens adjustment in response to minute variations in the position of the surface of the optical disc relative to the lens. Such variations are caused by thickness variations and non-flatness of the disc or turntable surface or apparatus vibrations. Since the discs rotate at high speeds (e.g., 1800 RPM), these focus-servo devices must respond at high frequencies (e.g., 500 Hz. for discs with ground glass substrates and in the range of 1000 to 3000 Hz. for discs with molded plastic substrates). Also, a subsystem to sense the focus error and an electronic feedback loop is required. Therefore these focus-servo devices are also costly, and fragile.
The costs and care of such sophisticated writing and reading apparatus do not present insurmountable problems for some industrial or laboratory applications, but they do present significant obstacles for consumer and business systems applications. As an alternative approach, it has been suggested to form master discs, with the sophisticated apparatus described above, and to replicate flexible read-only discs from the masters. Such flexible, read-only discs are rotated to fly on an air cushion and various devices are provided to positively constrain the disc's information surface toward a predetermined position relative to the focal point of the reading lens means.
U.S. application Ser. No. 160,769, entitled "Improved Optical Disc Method, Media and Apparatus for Writing and/or Reading Information", filed June 18, 1980, in the names of Geyer and Howe provides an alternative approach for writing and/or reading on flexible flying discs. In contrast to the constrained position approach described above, the Geyer and Howe approach flies flexible optical discs, in a dynamically stabilized condition, specifically avoiding localized constraint structures which force the disc to a predetermined position relative to the working lens means. Thus the Geyer and Howe approach substantially stabilizes the flying disc's neutral plane (the plane within a disc along which material is neither compressed nor extended during bending) in a plane of rotational equilibrium and the information surface of such disc has thereby been stabilized to an extent yielding unexpectedly precise writing and reading results. However the approach of the Geyer and Howe application still yields focus imprecision to the extent of thickness variations in the disc. That is, disc thickness variations cause different surface portions of the disc to be different distances from the disc's neutral plane; and, although the disc's neutral plane is stable relative to the focal point of the working lens, the disc's record surface moves in relation to that focal point by one-half the thickness variation ".DELTA.t" from a nominal disc thickness "t".